1. Field of the Invention
The present invention relates to a fuel electrode of a solid polymer electrolyte fuel cell. More particularly, the present invention relates to a fuel electrode of a solid polymer electrolyte fuel cell in which the reduction of the electrode performance is difficult to occur even in low fuel conditions caused by an interruption of fuel supply or the like.
2. Description of the Related Art
Fuel cells have high expectations for a next-generation power generation system. In particular, a solid polymer electrolyte fuel cell using a solid polymer electrolyte as an electrolyte is promising as a power supply for electric vehicles because it operates at relatively low temperatures compared with other types of fuel cells such as a phosphoric acid fuel cell and is compact.
FIG. 3 schematically shows the structure of a solid polymer electrolyte fuel cell. As shown in FIG. 3, the solid polymer electrolyte fuel cell 1 has a laminated structure consisting of two electrodes, a fuel electrode 10 and an air electrode 11, and a solid polymer electrolyte membrane 12 held tight between these electrodes. In addition, a dispersion layer 13 to supply fuel across both of the electrodes is provided for each of the electrodes, and a plurality of separators 14 are further installed to partition the cells of the fuel cell. In this solid polymer electrolyte fuel cell 1, the fuel consisting of hydrogen or methanol is supplied to the fuel electrode 10 to be oxidized to produce proton; oxygen or air is supplied to the other air electrode 12 to be reduced to oxygen ion; and the oxygen ion is combined with the proton that has passed through the solid polymer electrolyte membrane 12 from the fuel electrode 10 to form water, these reactions resulting in generation of electric power.
The solid polymer electrolyte fuel cells that satisfy requests such as efficient power generation performance or reduction of device size have been developed and are proceeding toward practical utilization. However, a new problem that the cell performance is reduced during fuel shortage in the operation of the fuel cell has been identified in recent years. More specifically, when abnormal conditions of fuel supply occur for some reason during the normal operation of the fuel cell, the fuel shortage reduces the electrode performance in the fuel electrode to reduce the cell performance, thereby interfering with the constant supply of electric power.
On the occurrence of such reduction of the electrode performance due to fuel shortage, the suspension of the electric power supply would be temporary and not critical if the performance could be restored by the re-normalization of the fuel supply. However, according to the previous reports, it has been recognized that the reduction of the electrode performance due to fuel shortage is irreversible and will not restore completely when the fuel supply is restarted.
Among the measures for these problems of the irreversible performance reduction due to fuel shortage, the establishment of a supply system in which the fuel supply never stops will be the first. However, even if improvement of such a peripheral system would be possible, it would be preferable to make improvements also to a fuel cell or an electrode itself that prevent the performance reduction during the fuel shortage, on the assumption of potential accidents.
As the measures for a fuel cell or an electrode itself, the improvements of catalysts composing electrodes or of the structure of electrodes have been reported. As the improvement of catalysts, for example, it is known to add ruthenium oxide (RuO2) or iridium oxide (IrO2) to a catalyst layer. As additional measures for the improvement, it is effective that the support to be applied is selected from the supports that have stable oxidation properties, such as graphitic carbon or titanium oxide (Ti4O7), and in addition that the loading of the catalyst particles on the support is to be increased (Refer to International Publications WO01/15247 and WO01/15254 for details of the improvement plans on these catalysts).
Further, as the measures in terms of the improvement of electrode structures, it is possible to minimize the performance reduction during fuel shortage by the procedure to increase the content of water in electrodes or by making it possible to suppress the discharge of water for the catalyst layer or the diffusion layer that composes electrodes (Refer to an International Publication WO01/15255 for details of the improvement plan).
However, according to the consideration of the present inventors, although these measures are effective, they are not sufficient, and measurable reduction of performance is identified during fuel shortage even when these measures are taken. Therefore, there is a need to look for further improvements different from these measures.
The present invention has been created under the background described above, and the object of the present invention is to provide a fuel electrode of a solid polymer electrolyte fuel cell in which the reduction of the electrode performance will not easily occur even when fuel shortage may be created.